It is well known that one of the major limitations in cellular and PCS wireless telephone networks is the so-called co-channel interference. In the case of TDMA networks, such as GSM or NADC (otherwise known as “IS-136”), co-channel interferences are often caused by the fact that the spectrum allocated to the system is reused multiple times (“frequency reuse”). The problem may be more or less severe depending on the reuse factor, but in all cases a signal, received by a handset, will contain not only the desired forward channel from the current cell, but also signals originating in more distant cells. If the interference from a distant cell causes a degradation of the ability of the handset to receive the desired signal correctly, it becomes important to identify the source of co-channel interference and measure the relative strength of interference relative to the desired signal.
It is important when performing a drive test of a wireless system to be able to separate signals that are coming from different base stations. Two phenomena make such a separation difficult: co-channel interference and adjacent-channel interference. When several base stations transmit on the same frequency, there are areas in the coverage region where conventional methods of power measurement are impractical or difficult to use when one needs to measure power from each of the interfering stations. This is also true for the case when stations operate on adjacent channels in close proximity.
A number of methods have been used to achieve the goal of signal separation. For example, drive-testing (measuring signal strengths with a scanning mobile receiver on board a test vehicle) in a system where each of the sectors uses a single unique frequency is described in U.S. Pat. No. 5,926,762.
Methods based on the association of signals with transmitting base stations based on the ability to decode the so-called “color codes” (base stations' IDs) have also been used. If a color code can be detected, the signal is ascribed to the nearest base station with this ID. Since color codes cannot be decoded using conventional receivers or handsets in presence of strong interference (co-channel or adjacent-channel), more advanced techniques of signal separation have been devised for color-code decoding when a signal, or signal component, is masked by interference.
One such technique of associating signals with base stations involves joint-decoding of the constituent signal components with channel estimation for each of the signal paths involved (described in U.S. Pat. No. 6,324,382, assigned to Agilent Technologies, Inc.). This method relies on an accurate estimation of the transmission channel characteristics for signal paths from each of the base stations contributing to the mixture of interferers at the reception site. Under conditions where the residual error of signal estimation due to the limitations of the complexity of channel modeling exceeds the level of weaker signals (or even the weaker of the two signals) and taking into account the realistic constraints of hardware complexity, the detection of the color code is all but impossible. The underlying reason for this result is that the color code embedded into the signal does not possess redundancy above what is normal for any digital code in the signal (in traffic and control channels), so that there is no additional processing gain when decoding color codes (BSIC in the case of GSM). Apart from poor decoding performance in practice, devices based on this approach suffer from slow scanning performance.
Another approach, described in U.S. Pat. No. 6,349,207, uses directional antenna arrays and time-space diversity to tune in a serial manner to one spatial signal component at a time with the exclusion, or at least attenuation, of the rest of the signal components. When an acceptable signal-to-noise ratio for a given interfering component is obtained, it is possible to demodulate and decode the color code corresponding to the station that transmitted the isolated component. This process is assisted by the detection of the interfering components in the signal by using correlations with known patterns (training sequences) in the signal. Knowing the number of components facilitates the time-spatial filtering algorithm. Although the described method apparently achieves the goal of associating interfering signal components with color codes and even with base station locations (by using RTOA-based triangulation), this technique requires complex and expensive equipment.
Another approach to the task of signal-component separation and signal identification is described in U.S. patent application Ser. No. 09/795,225 filed Feb. 28, 2001, now U.S. Pat. No. 6,931,235 and assigned to the assignee hereof. The Ser. No. 09/795,225 application is incorporated by reference herein, in its entirety, for all purposes. This approach is based on using correlation with known patterns in a signal (synchronization patterns and training sequences, for example), which yields a significant processing gain. This gain allows detection of the presence of an interfering component even when its level is substantially below the levels of interfering signals. Signal identification (i.e., association with transmitting stations) is based on the ability to track individual components during a drive test based on the knowledge of their respective times of arrival. By observing each of the detected components separately in the course of the drive test, one is able to relate the component to a geographical position where its contents, including the color code, can be easily and reliably determined. Then, by using the information logged in a data base for the whole life span of the component, all instances of the detection of this component are back-annotated with the BSIC value of the signal, or the name of the base station determined based on its geographical location at the moment of signal determination (being the closest station transmitting on the frequency channel when the component strength was at the maximum value).
The advantage of the correlation method is that it relies on a robust characteristic of the signal (correlation with a known pattern) that possesses processing gain. However, using for correlation only available fixed patterns has several drawbacks. They can be illustrated using the example of the GSM standard. The following fixed patterns can be used for the goal of signal component correlation: the frequency-correction burst, FCCH; the midamble of the synchronization burst, SCH; 8 distinct training sequences in the middle of traffic bursts, TCH.
The following observations can be made about the above patterns: FCCH is probably the most advantageous of the patterns for correlation due to its substantial length (a whole burst of about 160 symbols, including guard bits. The problem with the pattern is that it consists of all zero bits (a piece of a CW waveform when GMSK-modulated). Because of this, firstly, its autocorrelation function is triangular in shape and wide (300 symbols). This leads to a poor ability of the instrument to discern between closely-spaced (in time) signals coming from different base stations. Another manifestation of this phenomenon is that at low signal-to-interference or signal-to-noise levels, the apparent time-of-arrival of a signal has a significant time uncertainty, displaying jitter from measurement to measurement.
In view of the above, in order not to miss too many measurement results, one has to open the correlation time window wider, inviting more false correlations into the result. Secondly, since any run of several zeros in the signal will produce a measurable output spike from the correlator, and some of these spikes will fall into the assigned time windows for detected signals, the probability of false detects cannot be too low.
As for the SCH midamble, it has been specifically designed to possess very good cross-correlation properties with the signals, and its autocorrelation function is essentially contained in a single-symbol length. The only problem with this sequence is its relatively short length—64 symbols—that severely limits the dynamic range of the measurement.
The TCH training sequences are just 26 bits long and do not have the required dynamic range. Combining multiple copies of training sequences causes the measurement speed to slow to an unacceptable level.
Approaches based on sensing fixed patterns in received signals, for example, the FCCH burst used in GSM-type systems for frequency correction, have been disclosed in a patent application published Oct. 25, 2001 as No. 2001/0034208A1 entitled “Method and Apparatus for Co-Channel Interference Measurements and Base Station Color Code Decoding for Drive Tests in TDMA, Cellular, and PCs Networks” now U.S. Pat. No. 6,931,235, and No. 2004/0166809A1 published Aug. 26, 2004 and entitled “Method and Apparatus for Co-Channel Interference Measurements and Interference Component Separation Based on Statistical Signal Processing in Drive-Test Area” both of which are assigned to the assignee hereof. The '208 and '809 published applications are incorporated herein by reference. Notwithstanding the known approaches, there continues to be a need for a method and apparatus for determining signal strength, power levels of various signals throughout an area of interest that do not suffer from the limited dynamic range and poor cross and auto correlation properties.